Scanning technique for laser ablation

ABSTRACT

Improved methods of modifying target surfaces through ablation are disclosed resulting in a reduction of debris redeposition on the target surface.

This is a continuation-in-part application of copending U.S. Ser. No.07/953,607 filed Sep. 29, 1992, now abandoned.

FIELD OF THE INVENTION

The present invention relates to the field of laser modification oftarget surfaces through ablation and methods for smoothing surfacesusing lasers, as well as targets modified by such ablative techniques.

BACKGROUND OF THE INVENTION

The use of laser beams to modify surfaces is known. In the early 1980's,it was discovered that pulsed lasers emitting in the ultravioletfrequency range could affect a target surface through ablativephotodecomposition (APD). Later it was found that by using APD, layersof target material could be removed on the order of about one micron oftarget material per pulse.

It was further noted that APD did not significantly alter thecharacteristics of the newly exposed material immediately below theablated material. This phenomenon has been explained as being due to theUV lasher providing enough energy in a short enough period of time toactually break the covalent bonds of the polymeric target materialswithout heating the substrate. (See U.S. Pat. Nos. 4,417,948 and4,568,632). Further scanning techniques using APD are disclosed in U.S.Pat. No. 5,061,342.

Upon further investigation, it was found that certain materials, whenablated, created varying amounts of ablation debris, some of which wasredeposited upon the surface of the target material. It was: believedthat this redeposited debris could frustrate efforts to predictablyalter the ablated target surface.

Further, it was found that certain materials could not be as cleanlyetched as others. A method for ablating a target surface while alsoremoving the deposited and adhered debris from the target surface whileavoiding further debris accumulation is not known.

SUMMARY OF THE INVENTION

A novel method for ablating surfaces in a way that simultaneously clearsaway deposited debris and avoids subsequent debris accumulation has nowbeen determined. To obtain a desired resulting surface on a selectedtarget, the debris formed during the ablation process which becomesredeposited at, and adheres to the target surface must be removed fromthe target surface before the ablation process continues over theremainder of the target surface.

In accordance with the present invention, a method is disclosed forphotoablating a target surface comprising the steps of: a) directing abeam of pulsed UV radiation at a predetermined initial scanning area onthe target surface; b) scanning said beam in a direction away from saidpredetermined initial scanning area to a first edge of the targetsurface; c) rotating the target in increments; d) returning the beam tosaid predetermined initial scanning area on the target surface; e)scanning the beam in a direction away from said predetermined initialscanning area to the edge opposing said first edge of said targetsurface; and repeating steps c), d) and e) in sequence such that theentire target surface is scanned.

In further accordance with the present invention, a method is disclosedfor photoablating a target surface comprising the steps of: a) directinga beam of pulsed UV radiation at a predetermined initial scanning areaon the target surface; b) scanning said beam in a direction away fromsaid predetermined initial scanning area to a first edge of the targetsurface; c) rotating the target 180 degrees; d) returning the beam tosaid predetermined initial scanning area on the target surface; and e)scanning the beam in a direction away from said predetermined initialscanning area to the edge opposing said first edge of said targetsurface such that the entire target surface is scanned.

It is further thought that the invention of the present application isespecially useful for profiling, crosslinked, thermoset, thermoplasticor other materials including optically clear materials suitable for useas contact lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a cross-sectional view of a contact lens blank. Thedotted line is an exaggeration of the desired final toric surface.

FIG. 2 is a schematic diagram depicting a conventional scanningtechnique (edge-to-edge or EE) for comparative purposes.

FIG. 3 is a schematic diagram depicting the novel center-to edge (CE)scanning technique.

FIG. 4 is a diagram depicting a representative experimental set up forthe CE scanning technique

FIG. 5 is a photograph depicting the unetched surface of Ultem™ forcomparative purposes.

FIG. 6 is a photograph showing the surface of an Ultem™ (GeneralElectric, Pittsfield, Mass.) target etched using the EE technique.

FIG. 7 is a photograph showing the surface of an Ultem™ (GeneralElectric, Pittsfield, Mass.) target etched using the CE technique.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 1, a toric curve can be placed on a target contact lensblank by removing more material at the periphery of the optical zone. Itis understood to the skilled practitioner in the field that surfaces ofany configuration can be produced using the present invention and thatthe figures presented are for illustrative purposes only.

FIG. 2 shows the laser beam scanning a contact lens surface using anedge-to-edge (EE) technique whereby the laser is fixed and its beam isdirected first at one edge of the target surface at position t_(o). Astage holding the target surface is then moved across the path of thebeam until the beam reaches the opposing edge and the stage is at itsfinal position, t₁, thereby scanning the entire target surface.

In accordance with one embodiment of the present invention, FIG. 3 showsthe laser beam in a fixed position having its beam directed at thebisecting line of the lens. The stage holding the lens is then movedsuch that the laser scans the surface of the target from the bisectingline toward a first edge. The stage therefore travels from its initialposition, t_(o) to the position t^(1/2). The stage to which the lens isaffixed is then returned to its original position, t_(o). The beam isthen once again activated at the bisecting line of the lens, and thestage is moved from the position t_(o) to position t₁ such that the beamis scanned across the remaining target surface to the edge opposing saidfirst edge such that the entire target surface has been scanned.

It is understood that the stage may be held fixed and the beam movedacross the target surface. It is further understood that in a preferredembodiment after the stage is moved to position t_(1/2), the stage isreturned to position t_(o), rotated 180 degrees, and then scanned againto position t_(1/2) thereby scanning the entire target surface.

FIG. 4 depicts one preferred set-up for employing the present invention.The raw laser beam (1) is emitted from an excimer laser (2). The rawbeam is directed to a first scanning mirror (3) and may be directed to asecond scanning mirror (4). The raw beam is then directed to a movablebeam modification stage (5) which may comprise a focussing lens (6)which produces high fluence. The bear, modification station may alsocomprise a cylindrical lens (7) for producing beams of low fluence.After the beam has been modified, it is directed to the target station(8). The target station comprises a rotatable means (9) to which isattached a mounting means (10) to hold the target (11) in place.

In FIGS. 5 and 6, it can be plainly seen that a conventional EE scan hasleft debris visible at 45× magnification on the surface. However, it canbe plainly seen from the photograph in FIG. 7 that, with no alterationin the system set-up other that employing the center-to-edge (CE)technique, use of such CE technique has left a visibly cleaner surfacewith no ablation debris.

The results shown in FIG. 7 are remarkable since difficulty had beenpreviously encountered attempting to ablate various thermoplasticcompounds such as Ultem™ (General Electric, Pittsfield, Mass.) andRadel™ (Amoco, Atlanta, Ga.). As shown in these photos, the Ultem™surface has been cleanly ablated with no visible debris at 45×magnification using the CE technique described herein. FIG. 6 shows thesignificant ablation debris which remains after a conventional EE scanof the Ultem™ surface.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a new method of modifying opticalsurfaces to produce changes in their spherical, cylindrical or otherrefractive power, shape factor or other surface geometry. This newmethod is a modification of the procedures taught in U.S. Pat. No.5,061,342, the entire content of which is incorporated by referenceherein.

The method of the present invention employs UV radiation to ablatematerial from a target surface in order to produce a desired finalsurface on the target. Suitable target surfaces to be ablated includecontact lenses, contact lens blanks, molds used to make contact lenses,tools used to make such molds, and any means which either directly orindirectly imparts a desired predictable final spherical, cylindrical orother refractive power, shape factor or other surface geometry on anobject, such as a contact lens.

Varying amounts of material must often be removed from a target surfaceto produce a desired end result. For example, to produce toric surfaceson a contact lens, more material must be removed from the edges orperiphery than from the central region of the optical zone. (See FIG.1). Therefore, to produce the toric surface, a significant amount ofablated debris is created at the periphery of the optical zone, ascompared to the debris created when the central region of the opticalzone is scanned and its surface ablated.

Toric contact lenses are understood to be lenses which correct theinsufficient visual acuity caused by astigmatism. Such lenses have acylindric component of refraction, which refractivity is not constant inall planes passing through the optical axis, but has a maximumrefractivity in one plane and a minimum refractivity in another planeperpendicular to the first plane.

It was discovered that when the laser beam begins its scan on a lens toproduce a toric surface, significant ablated debris was randomlyredeposited on the surface of the lens. Some of the debris wasredeposited on the lens target in the path of the laser beam scan. Whenthis occurred, as the laser continued its scan, the first materialencountered by the beam was not the original target surface, but was theredeposited and freshly adhered debris from the periphery.

While the final surfaces created from the so called "edge-to-edge" (EE)scans described in U.S. Pat. No. 5,061,342 were often an improvementover other known surface modification procedures, such as lathing, etal., it was believed that the debris which was sometimes created,hindered the best results possible.

Therefore, as contemplated by the present invention, the beam isinitially located at a predetermined initial scanning area, for examplea bisecting line of the lens, and scanned toward a first edge of thelens. When the beam reaches the first edge, in one preferred embodiment,the scan is terminated and the beam returned to the predeterminedinitial scanning position. The stage to which the lens or other targetis affixed is then preferably rotated 180 degrees such that the beam maytravel in the same direction as it traveled to scan the first "half" ofthe target. At this point the beam once again is activated and the scanproceeds to move toward the edge opposing the first edge, this timescanning over the remainder of the target surface. In this way more ofthe debris seems to be effectively "swept" progressively forward andaway from the optical zone and does not appear to interfere with thedesired final surface.

It is understood that a bisecting line on the target surface is a lineextending across the center of the target surface. (The target surfaceneed not be circular or spherical in shape.)

Therefore, to achieve the improved ablation results in accordance withthis invention, the beam may be scanned in a direction proceeding from"center-to-edge" or from "edge-to-center" depending only upon what finalresults are desired in view of the condition of the initial targetsurface. In other words, the amount of material to be removed at anygiven point from the target surface determines where the beam scan willbegin and end.

It is therefore believed that to reduce debris accumulation on thetarget surface, the beam scan must begin at a point on the targetsurface where the least amount of material is to be removed and theleast amount of debris will be formed. It will therefore, be understoodby the skilled practitioner in the art, that the scans may begin and endin any variety of positions imaginable to create any finished surfaceeffect desired, from any shaped original target surface.

Further, the predetermined initial scanning area may be the bisectingline of the target, or may be a line or area which is between thebisecting line and an edge of the target surface. Therefore it iscontemplated that the present invention can be used to make sphericaland aspherical lenses including but not limited to toric, bifocal andmultifocal lenses. What is required, is that the beam be scanned from aninitial scanning area to one edge, followed by at least one subsequentscan from the initial scanning area in a direction outward to a secondedge, etc. in a fashion such that the entire target surface iseventually scanned. Therefore, it is contemplated that the presentinvention can be used to make standard and a spherical lense includingbut not limited to toric, bifocal, or multiple focal lenses.

For example, the stage may be rotated to any degree and scanned inrepeated increments such as, for example, every 20 degrees, until theentire target surface has been scanned. In the case of modifying thesurface of contact lenses, the target surface is generally the opticalzone which is about as wide as the raw beam generated by the excimerlaser.

Therefore, the scan of the entire target surface can be effected byrotating the stage 180 degrees and making one scan (or two CE scans)across the optical zone. However, the beam could be modified, as will bereadily apparent to those skilled in the art, to be so narrow as torequire rotating the target, for example, in 20 degree incrementsfollowing each scan from the bisecting line to the target edge.

It is understood that after scanning from the center to one edge in onedirection, that the inactivated beam could then be returned to thecenter and then made to scan in the opposite direction toward the edgeopposite the edge toward which the beam initially scanned. In this waythe entire target surface is scanned but the stage holding the target isnot rotated or indeed moved at all. This embodiment would be importantif the target being scanned could not be conveniently or accuratelyrotated, such as a supine patient having corneal surgery.

The laser energy applied to a target per unit area is known as thefluence, which for UV radiation is often expressed in terms ofmillijoules per square centimeter (mJ/cm²). The fluence range of thelaser scanned in accordance with the present invention is preferablyfrom about 20 to about 5000 mJ/cm², is more preferably from about 500 toabout 2000 mJ/cm², and is most preferably from about 750 to about 1500mJ/cm².

While the method of the present invention will work at any given energylevel it is understood by those skilled in the field that certainmaterials will require a certain fluence to effectively affect surfacecharacteristics through ablation.

A "crosslinked" polymeric material is understood to describe anypolymeric material which has any attachment of two chains of its polymermolecules by bridges comprised of either an element, a group, orcompounds known as crosslinking agents.

The term "thermoset" refers to polymeric material which solidifies or"sets" irreversibly when heated. By contrast, a "thermoplastic" materialis understood to refer to a polymer which softens when exposed to heatand is able to return to its original condition.

While the present invention is well suited for the modification ofcontact lenses, the modification of contact lens buttons, blanks andmolds, as well as the tools used to make the molds and the contactlenses is also contemplated by the present invention. Indeed any meansfor imparting optical properties or surface geometries may be modifiedby the present invention. The surface modification through laserscanning of such tools used to make the molds which, in turn, are usedto make contact lenses in a cast molding procedure is disclosed in aconcurrently filed and commonly assigned U.S. Ser. No. 07/953,425.

The present invention is further thought to be useful for other laserscanning applications such as corneal sculpting as well as any otherprocedures where ablation debris has been noted as an obstacle toachieving better target surface quality after scanning.

The following examples serve only to further illustrate aspects of thepresent invention and should not be construed as limiting the invention.

EXAMPLES 1-5 Toric Surfaces on Ultem™

Samples of Ultem™ were ablated using the scanning technique disclosed inU.S. Pat. No. 5,061,342 at a rate of about 140 pulses per sample. Thestandard fluence used was about 940 mJ/cm².

The samples were all inspected preliminarily with the unaided eye fortransmission and reflection with observations recorded. In each case the:samples were cleaned with Axarel™ (DuPont, Wilmington, Del.) althoughany suitable cleaning agent may be used as will be apparent to theskilled practitioner in the field. Photographs were then taken of thesamples followed by curve inspections using a ZYGO Mark IVInterferometer.

Further definitions are presented below: EE Scan--"Edge-to-edge"--singlescan across target beginning at one edge of the optical zone andcontinuing across target to the other edge of the optical zone.

CE Scan--"Center-to-edge"--Single scan beginning at center of opticalzone and continuing across to one edge of optical zone. The target isthen rotated 180 degrees, the beam is returned to its starting positionand repeats the scan.

EXAMPLE 1 Edge-to-Edge scan

samples of flat Ultem were scanned using a standard scanning profile forapproximately 140 pulses.

The results of the EE scan are shown at FIG. 6. Dark bands perpendicularto the scan occurred near the beginning of the scan. The bands wereeasily visible to the unaided eye in transmission and reflection. Aftercleaning with Axarel the black ring was completely removed, the darkband reduced but still evident.

EXAMPLE 2

The results of the CE scan are shown at FIG. 7. Slightly dark bandoccurred on the first scan side. The band was easier to view inreflection. After cleaning with Axarel the black ring was gone, the darkband was only slightly visible in transmission but not at all visible inreflection.

While the previous samples used in the examples were all flat, theexperiments were repeated for convex and concave samples withessentially the same results.

EXAMPLE 3 Roughness Comparison

Two flat Ultem samples were modified, one by CE and one by EE scanningtechniques. The resulting samples were analyzed by Talysurf surfaceprofilometer, The following definitions were used:

Sample 1--EE Scan

Sample 2--CE Scan

Full Zone--The entire etched area

Zone 1--Left of Center (Etched last on EE)

Zone 2--Right of Center (Etched first on EE)

    ______________________________________                                        Sample  Zone     Roughness* (Avg.)                                                                           Improvement                                    ______________________________________                                        1       Full     0.1378        --                                             2       Full     0.0774        44%                                            1       1        0.0614        --                                             2       1        0.0289        53%                                            1       2        0.1152        --                                             2       2        0.0351        70%                                            ______________________________________                                    

This data confirms a significant reduction in average surface roughnessusing the CE technique.

Many other modifications and variations of the present invention arepossible to the skilled practitioner in the field in light of theteachings herein. It is therefore understood that, within the scope ofthe claims, the present invention can be practiced other than as hereinspecifically described.

I claim:
 1. A method for photoablating a target surface comprising thesteps of: a) directing a beam of pulsed UV radiation at a predeterminedinitial scanning area on the target surface; b) scanning said beam in adirection away from said predetermined initial scanning area to a firstedge of the target surface; c) rotating the target in increments; d)returning the beam to said predetermined initial scanning area on thetarget surface; e) scanning the beam in a direction away from saidpredetermined initial scanning area to the edge opposing said first edgeof said target surface; and repeating steps c), d) and e) in sequencesuch that the entire target surface is scanned.
 2. A method forphotoablating a target surface comprising the steps of: a) directing abeam of pulsed UV radiation at a predetermined initial scanning area ofthe target surface; b) scanning said beam in a direction away from saidpredetermined initial scanning area to a first edge of the targetsurface; c) rotating the target 180 degrees; d) returning the beam tosaid predetermined initial scanning area of the target surface; and e)scanning the beam in a direction away from said predetermined initialscanning area to the edge opposing said first edge of said targetsurface, such that the entire target surface is scanned.
 3. The methodof claim 2 wherein said beam of pulsed UV radiation is emitted from anexcimer laser.
 4. The method of claim 2 wherein said target surface iscomprised of a crosslinked polymeric material.
 5. The method of claim 2wherein said target surface is a thermoset material.
 6. The method ofclaim 2 wherein said target surface is a thermoplastic material.
 7. Themethod of claim 2 wherein the fluence of said beam of pulsed UVradiation is from about 20 mJ/cm² to about 5000 mJ/cm².
 8. The method ofclaim 2 wherein the fluence of said beam is about 1 J/cm².
 9. The methodof claim 2 wherein said predetermined initial scanning area is abisecting line.
 10. The method of claim 2 wherein said target is acontact lens.
 11. A method for photoablating a target surface comprisingthe steps of: a) directing a beam of pulsed UV radiation at apredetermined initial scanning area of the target surface; b) scanningsaid beam in a direction away from said predetermined initial scanningarea to a first edge of the target surface; c) inactivating said beam;d) returning the beam to the predetermined initial scanning area; e)reactivating said beam; and f) scanning said beam in the directiondirectly opposite the scan made in step b) such that the entire targetsurface is scanned.
 12. A modified target surface prepared by ablativephotodecomposition comprising the steps of: a) directing a beam ofpulsed UV radiation at a predetermined initial scanning area of thetarget surface; b) scanning said beam in a direction away from saidpredetermined initial scanning area to the edge of the target surface;c) rotating the target 180 degrees; d) returning the beam to thepredetermined initial scanning area of the target surface; and e)scanning the beam in a direction away from said predetermined initialscanning area to the edge opposing the said first edge of said targetsurface, such that the entire target surface is scanned.
 13. The targetsurface of claim 12 wherein said target surface is a means capable ofimparting optical properties or surface geometries characteristics on asecond surface.
 14. The method of claim 12 wherein said target surfaceis a contact lens.
 15. A corneal target surface modified by ablativephotodecomposition comprising the steps of: a) directing a beam ofpulsed UV radiation at a predetermined initial scanning area of thetarget surface; b) scanning said beam in a direction away from saidpredetermined initial scanning area to a first edge of the targetsurface; c) inactivating the beam; d) returning the beam to thepredetermined initial scanning area of the target surface; and e)scanning the beam in a direction directly opposite from the direction ofthe scan in step b) from said predetermined initial scanning area to theedge opposing said first edge of said target surface, such that theentire target surface is scanned.
 16. The method of claim 1 wherein saidpredetermined initial scanning area is a bisecting line.
 17. The methodof claim 11 wherein said predetermined initial scanning area is abisecting line.
 18. The modified target surface of claim 12 wherein saidpredetermined initial scanning area is a bisecting line.
 19. The cornealsurface of claim 15 wherein said predetermined initial scanning area isa bisecting line.